In pay-per-view television systems, such as cable TV systems, which are operated by private broadcasters and funded by customer fees, it is desired to prevent unauthorized viewers from receiving particular programs. This is accomplished by transmitting programs in encoded form. Authorized customers are furnished with special decoders by the supplier of the programs for an appropriate fee. Such decoders, for example, may first evaluate a recognition code (designated herein as a signum) contained in the video signal, and only on recognition of the signum, enable the decoder to display the television image received in a decoded form.
In practice there exists the problems of being able to unequivocally recognize the signum despite corruption of the signal in order to allow authorized parties to receive the program but to disable the decoder if the signum is not present.
It is an object of the invention to create a process and circuitry which are capable of recognizing the signum even where signals suffer corruption.